Generally, in a mobile communication system, a mobile station uses a preamble which is broadcasted and used for searching for a cell to which the mobile station itself belongs. In order to effectively search cells using the preamble, various codes are used, which can be implemented by putting a limitation on a frequency or a spreading code, or the like, that is to be used.
In the above methods, a cell search performance depends on a sequence itself. As an example of a sequence having a good autocorrelation characteristic and peak to average power ratio (hereinafter abbreviated PAPR), there is CAZAC (constant modulus and zero autocorrelation) sequence. A mobile station is able to effectively search a cell using the CAZAC sequence. However, in an environment where other neighbor cells are simultaneously detected like a cell boundary area, the mobile station is unable to search a neighbor cell with simple algorithm but has to perform a full search by calculating correlation for a sequence corresponding to the entire cells.
FIG. 1 is a diagram of a cell search process according to a related art. Referring to FIG. 1, a cell transmits a preamble 11 to enable mobile stations to discriminate cells before transmitting data or control information. In case that a mobile station attempts to communicate with the cell, it receives the preamble 11 transmitted from the cell and then carries out channel estimation using the received preamble 11.
In order to discriminate cells by receiving a preamble, the mobile station is provided with a code set consisting of codes respectively corresponding to the cells and performs a cell search using signals transferred by a correlator 13 and correlation values. In the above cell search method, cell search algorithm should be simple to effectively perform the cell search.
FIG. 2 is a structural diagram of a frame applied to a communication system. First of all, assuming that a preamble code set transmittable by a cell is named Cp and that a preamble code assigned to a jth cell is named Cj, the cell transmits the preamble code before broadcasting control information or data and then additionally transmits information for enabling precise channel estimation or synchronization.
A receiving side obtains signals of a received preamble part and then checks a correlation value with a preamble set provided to the receiving side. Equation 1 related to a correlation value checking method.
                                          J            k                    ⁡                      (            τ            )                          =                              ∑                          τ              =              0                                      N              -              1                                ⁢                                    c              k                              n                *                                      ⁢                          c              j                              n                -                τ                                                                        [                  Equation          ⁢                                          ⁢          1                ]            
In Equation 1, Ck indicates an arbitrary code existing in a code set, N indicates a code length, and τ indicates a delay component of autocorrelation. In this case, a cost JMk(τ) for Ck can be expressed as Equation 2.
                              JM          k                =                              max                                          τ                =                0                            ,                                                          ⁢              …              ⁢                                                          ,                              N                -                1                                              ⁢                                    J              k                        ⁡                          (              τ              )                                                          [                  Equation          ⁢                                          ⁢          2                ]            
If the cost is calculated for each code constructing the code set and if a code having a maximum value among the calculated costs is selected, it is able to search a cell corresponding to the selected code. A cell search method using the cell cost is represented as Equation 3.
                              K          *                =                  a          ⁢                                          ⁢          r          ⁢                                          ⁢          g          ⁢                                          ⁢                                    max                                                k                  =                  0                                ,                …                ,                                  n                  -                  1                                                      ⁢                          JM              k                                                          [                  Equation          ⁢                                          ⁢          3                ]            
In case that a code itself fails to have any configuration, all the codes configuring the code set should be searched. Yet, in case that a code itself has a specific pattern, it is able to efficiently search a cell using it. For instance, in case that CAZAC sequence is used as a preamble code, it is able to use a linear frequency increase characteristic of the CAZAC sequence. The frequency increase characteristic shows a single frequency component if a progression of differences of a phase is calculated, if the calculated progression of differences is converted to an exponential function value, and if discrete Fourier transform (hereinafter abbreviated DFT) is performed on the exponential function value. Yet, a result of the DFT in the characteristics of the CAZAC sequence is totally changed according to the influence made by a different CAZAC sequence coming from another cell in the vicinity of a cell boundary. Accordingly, in case of using such a sequence as CAZAC, it is able to obtain a transmitted preamble code through simple algorithm if a peripheral interference signal does not exist. If an interference signal however exists, the characteristics of the CAZAC sequence are ruined. Accordingly, it is difficult to detect the preamble through simple algorithm.
In particular, in case that a mobile station searches all possible code combinations, it is unable to efficiently search a code sequence. In case of such a sequence having a specific characteristic as a CAZAC sequence, if interference signals considerably exist around a cell boundary, it is unable to search a code sequence using simple algorithm.